1. Field of the Invention
This application claims the benefit of priority to Japanese Patent Application No. 2004-000867 filed on Feb. 26, 2004, herein incorporated by reference.
The present invention relates to an intermediate frequency circuit suitable for a television tuner.
2. Description of the Related Art
A conventional intermediate frequency circuit is shown in FIG. 3. A first capacitor 44 and a first inductor 45 are connected in parallel between a pair of input ports 411, 412 of an intermediate frequency input circuit 41, and an intermediate tap of the first inductor 45 is connected to a power supply terminal 415. A parallel-connected circuit composed of a second capacitor 46 and a second inductor 47, a fourth capacitor 50 and a first resistor 51 are connected in series between one input port 411 and one output port 413, and a parallel-connected circuit composed of a third inductor 48 and a third capacitor 49, a fifth capacitor 52 and a second resistor 53 are connected in series between the other input port 412 and the other output port 414.
In addition, a collector of one transistor 54 in a frequency mixing circuit 42 is connected to one input port 411, and a collector of the other transistor 55 therein is connected to the other input port 412.
A base of one transistor 56 in an intermediate frequency amplifying circuit 43 is connected to one output port 413, and a base of the other transistor 57 therein is connected to the other output port 414.
The parallel-connected circuit composed of the first capacitor 44 and the first inductor 45 is tuned to an intermediate frequency, the parallel-connected circuit composed of the second capacitor 46 and the second inductor 47 forms a trap circuit which parallel-resonates at an intermediate frequency component of an upper-side adjacent channel, and the parallel-connected circuit composed of the third inductor 48 and the third capacitor 49 forms a trap circuit which parallel-resonates at an intermediate frequency component of a lower-side adjacent channel.
Further, an intermediate frequency signal output from the frequency mixing circuit is input to the intermediate frequency input circuit 41 through a pair of input ports 411, 412. In the intermediate frequency input circuit 41, the intermediate frequency signal is tuned by the parallel-connected circuit composed of the first capacitor 44 and the first inductor 45, unnecessary intermediate frequency component of the upper-side adjacent channel is removed by the parallel-connected circuit composed of the second capacitor 46 and the second inductor 47, and unnecessary intermediate frequency component of the lower-side adjacent channel is removed by the parallel-connected circuit composed of the third inductor 48 and the third capacitor 49.
Subsequently, the intermediate frequency signal is output to one output port 413 through the fourth capacitor 50 and the first resistor 51 and to the other output port 414 through the fifth capacitor 52 and the second resistor 53. The intermediate frequency signals from the pair of the output ports 413, 414 are input to the intermediate frequency amplifying circuit 43 (For example, see Patent Document 1).
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2001-196961(FIG. 4)
Generally, the RF characteristic of a television tuner (the transfer characteristic up to the frequency mixing circuit 42) is observed by connecting a detector having a low impedance to the intermediate frequency input circuit 41 so that the characteristic of the intermediate frequency input circuit 41 does not appear. However, the RF characteristic is still influenced by the parallel resonance circuit (trap circuit) provided for removing the intermediate frequency component of the adjacent channel.